Poster Presentation 20th Lancefield International Symposium on Streptococci and Streptococcal Diseases 2017

Selection of M protein deficient Streptococcus pyogenes: mechanisms and insights (#192)

Kristin K Huse 1 , Lucy Lamb 1 , Claire E Turner 1 , Matthew K Siggins 1 , Mark Reglinski 1 , Nicola N Lynskey 1 , Shiranee Sriskandan 1
  1. Imperial College London, London, UK, United Kingdom

Background. The streptococcal cell surface M protein is arguably the most studied virulence factor of S. pyogenes and is postulated to be essential for GAS survival in the human host through mechanisms which culminate in evasion of opsonophagocytic killing. Despite the central role ascribed to M protein in combatting clearance, we identified selection and emergence of M protein-defective S. pyogenes during experimental infection and set out to determine the mechanisms permitting this to occur.

Methods. Following a 24h infection, invasive isolates were whole genome sequenced. Additional isogenic strains with mutations in emm were constructed using standard techniques. Bacterial survival and uptake were assessed in human whole blood and neutrophil uptake assays respectively.

Results. Unexpectedly, following 24h experimental infection non-mucoid colonies demonstrated a mutation in emm (M’) that prevented surface expression of M protein. Only the parent emm1 S. pyogenes survived in whole blood compared to the M’ mutant and an engineered isogenic emm1 knockout strain. Co-incubation with soluble M1 protein (1 µg/ml) increased the multiplication of the emm1 negative strain to at least 20-fold greater than the WT. These results were corroborated using a panel of Lactococcus lactis strains: WT  L. lactis was readily phagocytosed by neutrophils whereas L. lactis expressing cell wall-anchored or soluble M1 protein  significantly reduced uptake by neutrophils.

Conclusion. The role of M1 protein is dual faceted and can rely on either cell wall-anchored or soluble forms. Our study elucidates the nuanced way in which M protein leads to the evasion of opsonophagocytosis.